Fix failing test introduced with r6111.

Test was assuming that getting the receive estimate of a stream which hasn't received packets would return an error, new behavior is to return 0.

TBR=wu@webrtc.org
BUG=crbug/371714

Review URL: https://webrtc-codereview.appspot.com/21419004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6114 4adac7df-926f-26a2-2b94-8c16560cd09d